The invention relates to an exhaust system for a multi-cylinder internal combustion engine.
Turbocharging an internal combustion engine, where supercharging is carried out by means of an exhaust-driven turbocompressor, has many advantages from the point of view of engine technology, and turbocharged engines are used increasingly frequently in cars.
One problem with modern turbocharged engines, however, is that it takes a relatively long time after starting to achieve good efficiency of a catalyst located downstream of the exhaust gas turbine. One reason for this is that the exhaust-gas turbine causes a drop in temperature when cold-starting. Furthermore, the narrow inlet at the turbine wheel results in high flow rates which lead to a high heat-transmission coefficient. The overall result is great thermal inertia upstream of the catalyst and, consequently, delayed ignition of the catalyst. Moreover, positioning the catalyst downstream of the exhaust-gas turbine leads to a relatively great distance between the engine and the catalyst. Reasons of space also make it usual to position the catalyst a long way from the engine, often under the floor of the vehicle.
The object of the invention is to produce an improved internal combustion engine from the point of view of exhaust gas. Another object is to achieve effective emission control more rapidly than previously when cold-starting.
These objects of the invention are achieved by means of an internal exhaust system in accordance with the invention.
By using a number of exhaust valves in each cylinder for distributing the exhaust-gas flow, it is possible to position a relatively small catalyst close to the engine and, during a starting phase, to guide all the exhaust gases to this catalyst before they reach an ordinary catalyst positioned downstream. The small catalyst starts functioning rapidly and, when the engine is hot, is exposed to only a proportion of the exhaust gases of the engine.
The solution according to the invention is particularly advantageous for turbo-engines because it means that exhaust gases do not have to be fed to the exhaust-gas turbine in connection with starting. As a result, the delay in effective emission control, which otherwise occurs in turbo-engines, is avoided.
The arrangement with a divided exhaust period also makes it possible to scavenge the engine more effectively than previously, with improved engine performance as a result.
Further features and advantages of the solution according to the invention emerge from the description and the other patent claims.